The CPU, obviously.
No thats false!
No thats false!
It's not. There are certain obstacles to upgrading the CPU (socket type, BIOS compatibility, cost effectiveness, etc...), but there are no inherent disadvantages to upgrading.
cpu matherboard memory
About 3-4 years. If you wait longer than that, it may be more convenient to replace the entire system, instead of upgrading the CPU.
Upgrading a CPU is best left to a professional,you should be in a ststic free enviorment.
Thermal grease to allow heat to transfer to the heat sink.
Confirm that the BIOS firmware support the dual core CPU
It is probably better to stick with the same manufacturer.
Depends upon the configuration
It is more expensive than a single CPU socket motherboard
If money is no concern, start by upgrading CPU. It helps more when running games with alot of data, but definately more to graphics card next